It is not an exaggeration to say that microfabrication of semiconductor integrated circuits has been accomplished by progress of photolithography and its peripheral technology. This photolithography is supported by broadly divided two technologies as well-known. One is a exposure wavelength and numerical apertures of a reduced projection exposure apparatus called a stepper and a scanner, and the other is a resist property which is mainly transfer resolution of a photoresist composition, where a mask pattern is transferred by the reduced projection exposure apparatus. These have interacted as if both wheels of a vehicle to improve processing precision of semiconductor integrated circuit patterns by photolithography.
A wavelength of a light source used for the reduced projection exposure apparatus has been shortened more and more because of request for high resolution of the circuit pattern. In general, at a resist resolution of about 0.5 μm, g-ray having 436 nm of a major spectrum of a mercury lamp is used; at about 0.5 to 0.30 μm, i-ray having 365 nm of the major spectrum of the mercury lamp is used; at about 0.30 to 0.15 μm, KrF excimer laser light of 248 nm is used; and at 0.15 μm or less, ArF excimer laser light of 193 nm is used. For more finely shortening, the use of F2 excimer laser light of 157 nm, Ar2 excimer laser light of 126 nm and EUV (extreme-ultraviolet radiation, wavelength 13 nm) has been investigated.
Meanwhile, with respect to photoresist compositions, at present, combined organic or inorganic anti-reflection films and innovated lighting systems are available, lifetime of photoresists for KrF is prolonged in lithography using the KrF excimer laser light, and development of photoresist compositions which bring about 110 nm of λ/2 or less has been conducted. In the lithography using the ArF excimer laser light, it has been desired to provide photoresist compositions for ArF, which is suitable for future mass production of about 90 nm nodes or less. The lithography using the F2 excimer laser light attracts the attention as one which takes on future microfabrication technology of 65 nm or less, and the development of photoresist compositions capable of significantly applying to the microfabrication by the lithography using this F2 excimer laser light has been also advanced.
As well-known, in the lithography, light with short wavelength is irradiated (exposure) on a photoresist layer applied on a laminated semiconductor substrate through a mask which reflects a negative or positive image pattern of the semiconductor integrated circuit pattern to be realized. The photoresist composition contains a photosensitive polymer as a major ingredient, which reacts with the irradiated light to become insoluble (negative) or soluble (positive) in alkali. Heating (post exposure bake, also abbreviated as “PEB”) to assure the reaction of the resist layer by the exposure after the irradiation of pattern light is conducted, and subsequently development is conducted to remove a soluble portion, thereby forming a photoresist pattern layer which accurately reflects the circuit pattern to be realized on the laminated semiconductor substrate. Thereafter, the patterned photoresist layer is sufficiently cured by heating (post bake) to give resistance to etching at next step in some cases. In the etching step, a surface layer or an upper layer of the laminated semiconductor substrate is dry-etched along the pattern using the patterned photoresist layer as the mask.
Therefore, major properties required for the photoresist composition are, first to obtain the resolution, therefore to have “transparency for irradiated light” where the pattern irradiated light can reach not only a surface portion of the resist layer but also a bottom portion of the substrate side and the resist layer can be significantly exposed over a whole thickness to the bottom portion, and to obtain the pattern with high resolution by an alkali developer by definitely differentiating an alkali soluble portion and an alkali insoluble portion after being irradiated with the pattern light. When the resist composition corresponding to the irradiated light with shorter wavelength than the above is developed, it is of course important to assure these major properties. High transparency at 157 nm which is a major spectrum of F2 excimer laser light is required for a base polymer of the resist composition usable for the lithography by the F2 excimer laser light which will become the light source of the stepper in next generation. However, existing resist materials have an absorbance just at this wavelength of 157 nm, i.e., no resist composition in the next generation can be obtained from the existing resist materials because the transparency is low for the irradiated light with a wavelength of 157 nm.
This way, in the technical field to provide the photoresist compositions, the development of a novel polymer having the high transparency at this wavelength of 157 nm is currently a problem. Up to now, the transparency for the irradiated light whose major spectrum is 157 nm is assured by introducing fluorine (F) and silicon (Si), as well as the development of novel polymers which combine resist performances such as alkali solubility, pattern transfer resolution and etching resistance, which affect a development property after the exposure has been advanced. As promising base polymers, numerous polymers such as fluorine-containing norbornene polymers (Non-patent Document 1 [M. K. Crawford, et al., “New Material for 157 nm Photoresists: Characterization and Properties” Proceedings of SPIE, Vol. 3999, (2000) pp 357-364] and Patent Document 1 (International Publication WO 00/67072 Pamphlet), fluorine-containing monocyclic polymers (Patent Document 2 [JP 2002-90997 A] and Non-patent Document 2 [Shun-ichi Kodama, et al., “Synthesis of Novel Fluoropolymer for 157 nm Photoresists by Cyclo-polymerization” Proceedings of SPIE, Vol. 4690, (2002) pp 76-83)]), tetrafluoroethylene copolymers (Non-patent Document 1 and Patent Document 1), and fluorine-containing monocyclic polymers (Non-patent Document 2) formed by cyclic polymerization of 1,1,2,3,3-pentafluoro-4-trifluoromethyl-4-hydroxy-1,6-heptadiene have been reported.
The fluorine-containing norbornene polymer (abbreviated as a conventional polymer A) disclosed in Patent Document 1 and Non-patent Document 1 and the fluorine-containing monocyclic polymer (abbreviated as a conventional polymer B) disclosed in Patent Document 2 and Non-patent Document 2 are believed to be promising as the base polymer for the photoresist composition in the next generation.
In the Non-patent Document 1, as specific examples of the conventional polymer A, a copolymer (abbreviated as a conventional polymer A1) composed of tetrafluoroethylene (49% by weight)/norbornene (51% by weight), a ternary copolymer (abbreviated as a conventional polymer A2) composed of tetrafluoroethylene (41% by weight)/norbornene (46% by weight)/vinyl acetate (12% by weight), a ternary copolymer (abbreviated as a conventional polymer A3) composed of tetrafluoroethylene (43% by weight)/norbornene (38% by weight)/vinyl acetate (20% by weight), a ternary copolymer (abbreviated as a conventional polymer A4) composed of tetrafluoroethylene (43% by weight)/norbornene (28% by weight)/vinyl acetate (29% by weight), a ternary copolymer (abbreviated as a conventional polymer A5) composed of tetrafluoroethylene (36% by weight)/norbornene (47% by weight)/t-butyl acrylate (17% by weight), a ternary copolymer (abbreviated as a conventional polymer A6) composed of tetrafluoroethylene (28% by weight)/norbornene (38% by weight)/t-butyl acrylate (34% by weight), and a ternary copolymer (abbreviated as a conventional polymer A7) composed of tetrafluoroethylene (42% by weight)/norbornene (41% by weight)/5-norbornene-2-carboxylate t-butyl ester (17% by weight) are disclosed.
This Non-patent Document 1 has taught that it is necessary to make an absorption coefficient (optical constant) of a resist film with normalized film thickness be 2.5 (μm−1) or less in order to obtain the sufficient pattern transfer resolution by the exposure light with a wavelength of 157 nm. The measured optical constants of the aforementioned polymers are 1.3 (conventional polymer A1), 2.0 (conventional polymer A2), 2.1 (conventional polymer A3), 2.6 (conventional polymer A4), 2.4 (conventional polymer A5), and 3.6 (conventional polymer A6), which are almost good values (conventional polymer A7 is not measured). Therefore, this conventional polymer A is acceptable as the base polymer of the next generation resist composition in terms of transparency when the laser light with a wavelength of 157 nm is exposed.
Meanwhile in Patent Document 1, as specific examples of the conventional polymer A, many examples are disclosed, and examples thereof may include a binary copolymer (conventional polymer A8) composed of tetrafluoroethylene (0.3 mol)/hexafluoroisopropanol substituted norbornene (0.2 mol) as the binary copolymer, and a ternary copolymer (conventional polymer A9) composed of tetrafluoroethylene (46 mol %)/hexafluoroisopropanol substituted norbornene (27 mol %)/OCH2C(CF3)2OCH2OCH3 substituted norbornene (27 mol %) as the ternary copolymer. The absorption coefficient of the conventional polymer A8 in the irradiated light with a wavelength of 157 nm is 1.27 μm−1 (film thickness 67.5 nm) and 1.40 μm−1 (film thickness 52.3 nm), which is preferable. The absorption coefficient of the conventional polymer A9 at a wavelength of 157 nm is 2.40 μm−1 (film thickness 69.2 nm) and 2.17 μm−1 (film thickness 54.9 nm), and the transparency of the both is acceptable as the base polymer for the next generation resist composition. Furthermore, in this Patent Document 1, using the resulting conventional polymer A as the base polymer, a photoresist composition was prepared, this was applied on a substrate to make a resist film, and a pattern was formed on this film to examine the pattern resolution. For example, in the photoresist composition using the conventional polymer A8 as the base polymer, 1.800% by weight of 2-heptanone, 1.648% by weight of cyclohexanone, 0.080% by weight of t-butyl lithocholate and 0.160% by weight of triphenylsulfonium triflate (5% by weight of cyclohexanone solution) were added to 0.312% by weight of conventional polymer A8. The resist film was formed from this photoresist composition, and the patterned light with a wavelength of 157 nm was irradiated to form a resist pattern. For the conventional polymer A9, using this polymer as the base polymer, the resist film was formed similarly, and the patterned light with a wavelength of 157 nm was irradiated to form a resist pattern. For the polymers of the other compositions, likewise, the formation of the resist film and patterning thereof were attempted. However, in Examples for patterning, it is not disclosed at all to evaluate how degree of the resolution was realized.
Therefore, in this Patent Document 1, although it can be confirmed that the transparency of the conventional polymer A is good for the light with a wavelength of 157 nm and that the conventional polymer A is promising as the base polymer of the photoresist composition for fine lithography using the irradiation light with a wavelength of 157 nm, the pattern resolution is not unknown, i.e., it is unknown whether a line width of the resist pattern required for the actual pattern of the semiconductor integrated circuit in the next generation has been accomplished or not and that if accomplished, whether the pattern shape is good or not.
However, it is described that the tetrafluoroethylene copolymer (conventional polymer A) disclosed in the above Non-patent Document 1 and Patent Document 1 is excellent in transparency for the wavelength of 157 nm, has plasma etching resistance, has a high glass transition point and is compatible with 0.26 N tetramethyl ammonium hydroxide developer commonly used, and thus the conventional polymer A is believed to be promising as the base polymer for the photoresist compositions in the next generation.
On the other hand, the conventional polymer B disclosed in Patent Document 2 is a fluorine-containing monocyclic polymer containing a repeat unit of a cyclic structure where a monomer unit (a) of a diene type monomer composed of a compound represented by the following formula (9) or a derivative thereof and a monomer unit (b) of a fluorine-containing vinyl monomer are cyclized, and having a blocked acidic group derived from the fluorine-containing vinyl monomer.
CH2═CH—X—CH═CH2 (9), wherein X represents a methylene group or an oxygen atom. It is described that the derivative of the above (a) contains an alkyl substituent and a hydroxyl group substituent, and that the substituted alkyl group is preferably a lower alkyl group having 1 to 4 carbon atoms.
In Patent Document 2, as specific examples of the conventional polymer B, four synthesis examples are disclosed.
In the synthesis example 1, 13.6 g of CH2═CHCH2CH═CH (represented by a monomer 1), 136.0 g of CF2═CFOCF2CF2C(CF3)OCOC(CH3)3 (represented by a monomer 2) and 10 mL of 10% by weight of diisopropyl peroxycarbonate in trichlorotrifluoroethane solution were added into 0.3 L of trichlorotrifluoroethane solvent, heated and polymerized to yield 17.4 g of the conventional polymer B (represented by a conventional polymer B1). In this conventional polymer B1, a ratio of the monomer 1 unit/monomer 2 unit is 35/65 (mol %) and a number average molecular weight in terms of polystyrene is 10,200.
In the synthesis example 2, 14.0 g of CH2═CHOCH═CH2 (represented by a monomer 3), 36.08 g of the monomer 2 and 10 mL of 10% by weight of diisopropyl peroxycarbonate in trichlorotrifluoroethane solution were added into 150 g of trichlorotrifluoroethane solvent, heated and polymerized to yield 12.2 g of the conventional polymer B (represented by a conventional polymer B2). In this conventional polymer B2, a ratio of the monomer 3 unit/monomer 2 unit is 31/69 (mol %) and a number average molecular weight in terms of polystyrene is 14,500.
In the synthesis example 3, 16.8 g of CH2═CHCH(OH)CH═CH2 (represented by a monomer 4), 150.5 g of the monomer 2 and 10 mL of 10% by weight of diisopropyl peroxycarbonate in trichlorotrifluoroethane solution were added into 150 g of trichlorotrifluoroethane solvent, heated and polymerized to yield 10.8 g of the conventional polymer B (represented by a conventional polymer B3). In this conventional polymer B3, a ratio of the monomer 4 unit/monomer 2 unit is 38/62 (mol %) and a number average molecular weight in terms of polystyrene is 12,300.
In the synthesis example 4, 13.6 g of the monomer 1, 117.9 g of CF2═CFOCF2CF2C(CF3)(CH3)OCOC(CH3)3 (represented by a monomer 5) and 10 mL of 10% by weight of diisopropyl peroxycarbonate in trichlorotrifluoroethane solution were added into 150 g of trichlorotrifluoroethane solvent, heated and polymerized to yield 9.4 g of the conventional polymer B (represented by a conventional polymer B4). In this conventional polymer B4, a ratio of the monomer 1 unit/monomer 5 unit is 39/61 (mol %) and a number average molecular weight in terms of polystyrene is 11,800.
Furthermore in this Patent Document 2, using the resulting conventional polymer B as the base polymer, a photoresist composition was prepared and applied on a substrate to make a resist film, and a pattern was formed on this film to examine the pattern resolution. That is, 100 parts by weight of each conventional polymer B1 to B4 and 5 parts by weight of trimethylsulfonium triflate were dissolved in 700 parts by weight of propyleneglycol monomethylether acetate to obtain a resist composition, this composition was uniformly applied on a silicon substrate by spin coating, and heated at 80° C. to obtain a resist film of 0.3 μm (300 nm). Values of a transmittance of the irradiation light with a wavelength of 197 nm through the resist films corresponding to the conventional polymers B1 to B4 are described to have been 72%, 68%, 65% and 71%, respectively. For the resolution thereof, it is described that dimensions of 0.25 μm (250 nm), 0.25 μm (250 nm), 0.24 μm (240 nm) and 0.24 μm (240 nm) were possible in line and space pattern.
This way, in Patent Document 2, the resolution of the line and space pattern with 0.24 to 0.25 μm (250 nm) at a resist film thickness of 0.3 μm (300 nm) has been obtained, but no exposure by the light with a wavelength of 157 nm of F2 excimer laser was conducted, and the transparency (optical constant) for the light with a wavelength of 157 nm and the resolution of the resist pattern have been unknown. An assumed resist film thickness when using the F2 excimer laser is 120 to 150 nm, and the desired resolution when the line and space pattern is formed is 150 nm or less, and preferably 100 nm or less. With respect to a desired transparent degree of the resist film, as described in the Non-patent Document 1, it is necessary to make an absorption coefficient (optical constant) of a resist film with normalized film thickness be 2.5 (μm−1) or less in order to obtain the sufficient pattern transfer resolution by the light exposure with a wavelength of 157 nm.
Therefore, in this Patent Document 2, although it can be confirmed that the transparency of the conventional polymer B is preferable for the light with a wavelength of 193 nm and that the conventional polymer B is promising as the base polymer of the photoresist composition for fine lithography using the irradiation light with a wavelength of 193 nm, the pattern resolution is not unknown, i.e., it is unknown how degree of the transparency for the irradiation light with a wavelength of 157 nm required for the actual pattern of the semiconductor integrated circuit in the next generation is, whether the line width of the desired resist pattern can be accomplished by this irradiation light with a wavelength of 157 nm, and that if accomplished, whether the pattern shape is good or not.
In Non-patent Document 2, for the fluorine-containing monocyclic polymer formed by cyclic polymerization of 1,1,2,3,3-pentafluoro-4-trifluoromethyl-4-hydroxy-1,6-heptadiene, the absorption coefficient, Tg, solubility in the developer and solubility in the resist solvent were evaluated, it was confirmed that such properties were preferable, and the resist composition was prepared to form the resist pattern of 100 nm.
From these results, it can be confirmed that the polymer is promising as the base polymer of the photoresist composition for the fine lithography using the irradiation light with a wavelength of 157 nm, as is the case with the conventional polymer A.
On the other hand, in Patent Document 1, it is described to add publicly known dissolution inhibitor such as tert-butyl ester of lithocholic acid.
However, even though the resist patterns of 90 nm and 80 nm may be resolved using the above conventional polymer A or B and the certain dissolution inhibitor in combination, the resist pattern shape is insufficient because a resist top portion becomes round. As described in the conventional art, practical application of the resist pattern of 70 nm or less is aimed because the lithography using the F2 excimer laser attracts the attention as one which will take on the micro fabrication technology of 65 nm or less in future. However, in the resist pattern of 80 nm or less, roundness of the resist top portion will be further worse. Therefore, it is an important subject to solve the problem.
On the other hand, it is described that the tetrafluoroethylene copolymer (conventional polymer C) disclosed in the above Non-patent Document 1 and Patent Document 1 is excellent in transparency for the wavelength of 157 nm, has plasma etching resistance, has a high glass transition point and is compatible with 0.26 N tetramethyl ammonium hydroxide developer commonly used, and thus it is believed to be promising as the base polymer for the photoresist compositions in the next generation.
In the Non-patent Document 1, as specific examples of the conventional polymer C, a copolymer (conventional polymer C1) composed of tetrafluoroethylene (49% by weight)/norbornene (51% by weight), a ternary copolymer (conventional polymer C2) composed of tetrafluoroethylene (41% by weight)/norbornene (46% by weight)/vinyl acetate (12% by weight), a ternary copolymer (conventional polymer C3) composed of tetrafluoroethylene (43% by weight)/norbornene (38% by weight)/vinyl acetate (20% by weight), a ternary copolymer (conventional polymer C4) composed of tetrafluoroethylene (43% by weight)/norbornene (28% by weight)/vinyl acetate (29% by weight), a ternary copolymer (conventional polymer C5) composed of tetrafluoroethylene (36% by weight)/norbornene (47% by weight)/t-butyl acrylate (17% by weight), a ternary copolymer (conventional polymer C6) composed of tetrafluoroethylene (28% by weight)/norbornene (38% by weight)/t-butyl acrylate (34% by weight), and a ternary copolymer (conventional polymer C7) composed of tetrafluoroethylene (42% by weight)/norbornene (41% by weight)/5-norbornene-2-carboxylate t-butyl ester (17% by weight) are disclosed.
This Non-patent Document 1 has taught that it is necessary to make an absorption coefficient (optical constant) of a resist film with normalized film thickness be 2.5 (μm−1) or less in order to obtain the sufficient pattern transfer resolution by the exposure with a wavelength of 157 nm. The measured optical constants of the aforementioned polymers are 1.3 (conventional polymer C1), 2.0 (conventional polymer C2), 2.1 (conventional polymer C3), 2.6 (conventional polymer C4), 2.4 (conventional polymer C5), and 3.6 (conventional polymer C6), which are almost preferable values (conventional polymer C7 is not measured). Therefore, this conventional polymer C is acceptable as the base polymer for the next generation resist composition in terms of transparency when the laser light with a wavelength of 157 nm is exposed.
Meanwhile in Patent Document 1, as specific examples of conventional polymer C, many examples are disclosed, and examples thereof may include a binary copolymer (hereinafter, described as conventional polymer C8) composed of tetrafluoroethylene (0.3 mol)/hexafluoroisopropanol substituted norbornene (0.2 mol) as the binary copolymer and a ternary copolymer (hereinafter, described as conventional polymer C9) composed of tetrafluoroethylene (46 mol %)/hexafluoroisopropanol substituted norbornene (27 mol %)/OCH2C(CF3)2OCH2OCH3 substituted norbornene (27 mol %) as the ternary copolymer. The absorption coefficient of the conventional polymer C8 in the irradiated light with a wavelength of 157 nm is 1.27 μm−1 (film thickness 67.5 nm) or 1.40 μm−1 (film thickness 52.3 nm), which is preferable. The absorption coefficient of the conventional polymer C9 at a wavelength of 157 nm is 2.40 μm−1 (film thickness 69.2 nm) or 2.17 μm−1 (film thickness 54.9 nm), and the transparency of the both is acceptable as the base polymer for the next generation resist composition. Furthermore, in this Patent Document 1, using the resulting conventional polymer C as the base polymer, a photoresist composition was prepared, this was applied on a substrate to make a resist film, and a pattern was formed on this film to examine the pattern resolution. For example, in the photoresist composition using the conventional polymer C8 as the base polymer, 1.800% by weight of 2-heptanone, 1.648% by weight of cyclohexanone, 0.080% by weight of t-butyl lithocholate and 0.160% by weight of triphenylsulfonium triflate (5% by weight of cyclohexanone solution) were added to 0.312% by weight of the conventional polymer C8. The resist film was formed from this photoresist composition, and the pattern light with a wavelength of 157 nm was irradiated to form a resist pattern. For the conventional polymer C9, using this polymer as the base polymer, the resist film was formed similarly, and the light with a wavelength of 157 nm was irradiated to form a resist pattern. For the polymers of the other compositions, likewise, the formation of the resist film and patterning thereof were attempted. However, in the Examples for patterning, it is not disclosed at all to evaluate how degree of the resolution was realized.
Therefore, in this Patent Document 1, although it can be confirmed that the transparency of the conventional polymer C is preferable for the light with a wavelength of 157 nm and that the conventional polymer C is promising as the base polymer of the photoresist composition for fine lithography using the irradiation light with a wavelength of 157 nm, the pattern resolution is not unknown, i.e., it is unknown whether a line width of the resist pattern required for the actual pattern of the semiconductor integrated circuit in the next generation has been accomplished and that if accomplished, whether the pattern shape is good or not.
In Non-patent Document 2, for the fluorine-containing monocyclic polymer (hereinafter, described as conventional polymer D) formed by cyclic polymerization of 1,1,2,3,3-pentafluoro-4-trifluoromethyl-4-hydroxy-1,6-heptadiene, the absorption coefficient, Tg, solubility in the developer and solubility in the resist solvent were evaluated, it was confirmed that such properties were preferable, and the resist composition was prepared to form the resist pattern of 100 nm.
From these, it can be confirmed that the polymer is promising as the base polymer of the photoresist composition for the fine lithography using the irradiation light with a wavelength of 157 nm, as is the case with the conventional polymer A.
On the other hand, in the following Patent Document 3 (JP 2003-2925 A), it is described to add a publicly known nitrogen-containing compound to a resist for F2.
However, even though the resist patterns of 90 nm and 80 nm may be resolved using the above conventional polymer C or D (hereinafter, conventional polymers C and D and the conventional polymers A and B are collectively referred to as “conventional polymers”) and the certain nitrogen-containing compound in combination, the resist pattern shape is insufficient because a resist top portion becomes round. As described in the conventional art, practical application of the resist pattern of 70 nm or less is aimed because the lithography using the F2 excimer laser attracts the attention as one which will take on the microfabrication technology of 65 nm or less in future. However, in the resist pattern of 80 nm or less, roundness of the resist top portion further worsens. Therefore, it is an important subject to solve the problem.
On an inorganic substrate having nitrogen-containing film such as SiON and a substrate provided with an organic anti-reflection film, the resolution and the pattern shape are insufficient in some cases.